1. Field of the Invention
The present invention relates to an electrophotographic image recording apparatus for visualizing an image by using toner, such as printers, facsimile machines and copying machines. More particularly, the invention relates to a developing method in a developing process for forming a toner image on a surface of a recording medium, and a recording apparatus using the method.
2. Description of the Related Art
The electrophotographic image recording apparatus uses a developing step and a fixing step. The developing step develops a latent electrostatic image into a toner image by supplying an image visualizing matter, called toner, to the latent electrostatic image. This latent image is formed on an image carrying member, called a photosensitive member, which rotates in one direction. The latent electrostatic image has an image portion having a predetermined potential Vr and a background portion having a predetermined potential V0. The fixing step fixes the thus formed toner image onto a recording medium.
A developing unit using a two-component developer consisting of toner and magnetic powder called carrier is frequently used in the developing unit for the electrophotographic system.
In the developing process, the developer is confined and agitated within the developing unit. The toner is charged through its friction with the carrier during the agitation. A method, called a bias developing method, is usually used for an image forming step of the developing process. A bias voltage Vb, which is in amplitude between the image portion potential Vr and the background portion potential Vo, is applied to a magnetic roller, called a developing roller, which is for transporting a developer to a location facing a latent electrostatic image on the photosensitive member surface of the developing unit. In forming an image, toner particles, charged under an electric field developed between the latent image formed on the photosensitive member surface and the developing roller, are separated from the developer and the separated ones migrate to the photosensitive member surface. A difference between the bias voltage Vb to the developing roller and the image portion potential Vr is called a developing potential difference. A difference between the bias voltage Vb and the background portion potential Vo is called a background potential difference.
If the developing potential difference is large, an electric field (referred to as a "developing electric field") for causing toner particles to migrate from the developing roller to the image portion on the photosensitive member is large, the developing performance is good. If the background potential difference is large, an electric field (referred to as a "cleaning electric field") for causing toner particles to migrate from the background portion on the on the photosensitive member to the developing roller is large, the amount of toner particles (referred to as "fog") attached to the background portion is reduced. Usually, those potentials Vr, Vb and Vo are selected in value so as to secure the developing performance high enough to obtain a preset image density, and to put the fog within a target level. With increase of the background potential difference, a phenomenon (referred to as "back-end bleach") in which the trailing end of the image as viewed in the rotational direction of the developing roller is insufficiently developed, and an image defect caused by scattering of carrier are likely to occur.
As the carrier particle diameter increases, a space filling rate of the developer in the developing portion is increased. As a result, the developing performance is improved to provide high quality images. Further, the surface area contributing to the friction charging of toner increases. Accordingly, the increase of the carrier particle diameter is also advantageous in that the charging stability of the developer is improved. However, with decrease of the carrier particle diameter, the carrier more easily scatters. To avoid this, the lower limit of the carrier particle diameter is determined within a range where the image defect by the carrier scattering does not occur.
There is a modification of the bias developing method for electrophotographic system. The modification follows. As known for long (see JP-A-48-37148, for example), a medium potential is provided which is in level between the potentials of the charging area and the discharging area on the photosensitive member. A first developing unit for carrying out a reverse developing is provided in the discharging area, and carries out the development by using first color toner. A second developing unit for subsequently carrying out a normal development is provided in the charging area, and carries out a development by using second color toner. Thus, the developing method of the modification carries out the development of two colors by one charging step and one light irradiation step (exposure step). In this developing method, no toner is attached, for development, to a medium potential area on the photosensitive member, which has a potential value which is between a bias voltage value of the first reverse developing unit and a bias voltage value of the second normal developing unit. A white image portion is formed in the medium potential area. Accordingly, the background portion of the white image, and two images by the first and second color image forming portions are formed. In the specification, the two-color developing method will be referred to as a potential-dividing developing method. The potential-dividing developing method is valid in principle if the first development is the normal development and the second development is the reverse development.
In the potential-dividing developing method, the following problem arises: a fringe of a certain color image in which no image should be present is developed with another color toner (this phenomenon will be referred to as a "fringe development").
A method to remove the fringe development has been proposed in JP-A-10-39573. In this method, auxiliary light is irradiated onto an area in which the fringe development will occur, whereby the reverse electric field occurring thereat is suppressed in magnitude (This method will be referred to as an "auxiliary exposure method").
Problems of the image forming apparatus using the conventional potential-dividing developing method will be described with reference to FIG. 6A. FIGS. 6A and 6B graphically represent a potential distribution and an electric field distribution with respect to positions on a photosensitive member surface after an exposure process is carried out. In the conventional image forming apparatus using the potential-dividing developing method, with respect to image portion potentials Vca and Vda having predetermined values, and the medium potential Vw having also a predetermined value, potential values Vb1 and Vb2 are selected such that a difference between Vw and Vb1 and a difference between Vw and Vb2 are relatively large in order to suppress the fringe development occurrence and to reduce the fog by increasing the background portion potential Vo. For this reason, the development potential differences (potential differences between the image portion potential Vca and Vb1 and between the image portion potential Vda and Vb2) are suppressed, and hence the developing electric field cannot be increased to be large. As a result, it is impossible to secure a sufficient developing capability.
A method to increase the developing capability, which is generally used, is a method by increasing the number of developing rollers. When the number of developing rollers is simply increased, in the image forming apparatus using the potential-dividing developing method, the following problem arises: a chance of the occurrence of the fringe development is increased, although the developing capability is increased